When in handling syringes or dripsets for medical administering purposes or during production or transit of such devices, there are a variety of possibilities that solid impurities can be admitted into the syringes or dripsets or admixed to the medical solution or blood which has been introduced into or stored in the devices. These impurities include not only dust or fine particles which have ingressed into the syringe or dripset from external sources but fine fragments of glass or rubber which are produced when an ampoule or other breakable container of a medical solution is opened or during the process in which a rubber plug is fitted on the fluid reservoir or a fluid conducting tube of a dripset prior to administration. The impurities thus present in a medical solution in a syringe or a dripset find their way through the needle holder and the needle into the bloodstream or body tissues of the human body and may injure the vascular tissues.
Medical administering injectors having filter media have therefore been proposed and put into use for the purpose of removing impurities from the medical solution or blood for intravenous transfusion before the solution or blood is discharged from the injection needles or cannulae of the administering devices. The filter medium is ordinarily located within the needle holder of a syringe or of the needle assembly of a dripset for collecting impurities from the medical solution or blood for transfusion at the substantial terminal end of the flow of the fluid through the needle assembly. In some cases, the filter media are located within the barrel of syringes or of adapters of dripsets for collecting impurities before the solution or blood is allowed into the needle holders.
Impurities contained in medical solutions of blood to be dealt by medical administering injectors in general usually range in respect of particle size from the order of microns to the order of hundreds of microns, as is well known in the art. For the purpose that impurities with particle sizes of the order of microns be collected by a filter medium satisfactorily, the filter medium must have voids that measure tenths of microns in diameter or width. If a filter medium having such a density is formed to the thickness required to provide a desired filtering ability and is incorporated in a needle assembly, the flow of the fluid to be passed through the filter medium is subjected to a resistance of a considerable degree and forces the filter medium to move forward in the needle assembly. In the process of injection, the filter medium is thus finally forced against the annular wall at the formost end of the chamber or passageway within which the filter medium is located. When this occurs, the filter medium has its circumferential annular portion covered by the inner face of the annular wall of the needle holder or the barrel of the needle assembly and is allowed to be effective to function as the filter medium only through its central portion coextensive with the cross sectional area of the inlet end of the passageway extending forward from the inner end face of the annular wall which is integral with a tip portion carrying the injection needle (if the filter medium is mounted in the needle holder) or with a tip portion connected to the needle holder (if the filter medium is located within the barrel of the needle assembly). The filter medium in this condition is permitted to exhibit only part of its potential ability and invites deterioration in the collection efficiency. In the case of a dripset, this may cause a failure of the dripset to feed the medical solution into the vein at a desired rate.
Another problem encountered by a filter medium for use in a medical administering injector is that the smaller the voids in the filter medium, the earlier the voids become clogged with solid particles collected by the filter medium. This also gives rise to reduction in the collection efficiency of the filter medium in the process of administration.